Disclaimer: There will be some romances listed that I have issues with. This does not mean said romances don't have good parts, they just aren't for me.
- No therapy. It's alright for the LI to have problems, but for the sake of every kitten in the world, don't have them whine about it to me every sentence. I want to see both characters supporting and helping each other, whether it's Garrus's heartfelt "I'm here if you need me." or Isabela's angry "There are other people who care about you, you know." My character does not want to feel like everyone's Living Emotional Crutch; they want to be supported, too.
- Equality. I'm not fond of romances where one character has more power than the other. I don't mean "badassery" or physical or mental or space magic power, I mean "authority" power. If it's done right it can be sweet, yes, but done wrong and it feels like one of them is blackmailing or otherwise forcing the other to be with them. One of the problems I have with Quinn's romance in SWTOR is how "yes my lord, whatever you say my lord" he is to the female Warrior. I get that's how he is, but it just feels a little abusive to me.
- Mental stability. Especially after DAII and it's list of asylum escapees, excluding Isabela. I don't want Jaesa's "I planted a tracking device on you to see if you were cheating on me", I don't want Anders's "I'd drown us both in blood to keep you safe", I don't want Merrill's "You can't save me. No one can save me." Give me sane, mentally healthy people to romance, Bioware. Please.
- Good transition from friendship to love. This isn't a Bioware example, but Skyrim was pretty much "blatantly hit on someone -> get married -> leave them at home and see them once in a while." I want to know someone before I hook up with them, and that means dating. Since this isn't a dating sim, that actually translates befriending them first. Garrus and Tali especially did this well, as did Zevran. The ME1 romances seemed to move a little too fast, particularly Liara. Oh, and Jacob from ME2, whose every conversation with a remotely-Paragon Shepard involved her hitting on him (not to mention THE PRIIIIIIIIIIIIZE).
- Room for error. This goes back to Baldur's Gate and Jaheira's ridiculously easy-to-screw-up romance. Bioware has gotten a lot better at giving you room to roleplay...except for ME2, which gave you about one chance to start each characters' romance, and SWTOR, where if you miss too many flirts you get locked out, or in some cases just one flirt (notably Ashara and Corso).
- Plot importance. I don't mean the LI has to be central to the story, but contributing to it in some way. Remember regular KOTOR and how Bastila turned to the dark side? Or Morrigan's Dark Ritual, Alistair's claim to the throne, Aribeth's betrayal? Those were all good. They happened regardless of the romance, but both the player and the character were more invested in those events because of the romance (that said, this is probably the least of all my "wanted traits", since there have been several non-plot romances so far that were very good).
- And finally, an actual feel of romance! By this I mean, little things you can do with your LI that feel "couple-like", or people commenting on the romance. Origin's option to kiss your lover was good, as was the various party-banter about said romance. Everything in the Citadel DLC, Garrus's date with FemShep in vanilla 3, and the various flirt-packs for BG and NWN made by the community are also good examples. Pretty much every other Bioware game had the romance exist only in your conversations and the achievement.
Out of all Bioware romances, the Garrusmance has to be my favorite because it nails every single one of these (except possibly Room for Error, and that's only because you could only start the romance in 2). And because Garrus is just awesome in general.